1. Field of the Invention
This invention relates to a method of controlling a rolling apparatus for rolling material to a predetermined thickness through a plurality of rolling passes and, more particularly, to a method of controlling a rolling apparatus such as to obtain optimum rolling with given rolling conditions.
2. Description of the Prior Art
To obtain efficient rolling of material to a desired thickness, it is necessary to set a rolling schedule in correspondence to various control parameters such as the gap and rotational speed of the roller for each rolling pass, thickness and temperature of the material, a motor torque for driving each roller, etc.
It is well known in the art that where a material is continuously rolled through a plurality of rolling passes, an i-th pass of coarse rolling process has a following equational relationship between the thickness H.sub.i of the material, the thickness h.sub.i of the material at the roller outlet, the rolling speed V.sub.i and the rolling force F.sub.i EQU F.sub.i =f.sub.i (H.sub.i, h.sub.i, V.sub.i) (1)
The motor power PW.sub.i required for executing the i-th pass is given as EQU PW.sub.i =f.sub.2 (F.sub.i, V.sub.i) (2)
The rolling speed V.sub.i is set such that V.sub.MIN .ltoreq.V.sub.i .ltoreq.V.sub.MAX wherein V.sub.MAX and V.sub.MIN are respectively the maximum speed and minimum speed and that the power PW.sub.i is less than the maximum permissible value. Once the thickness h.sub.i and rolling force F.sub.i are determined, the position or gap of roller is calculated according to an equation EQU S.sub.i =f.sub.3 (F.sub.i, h.sub.i) (3)
Once these control parameters are determined, the required number of passes and the minimum time required for the completion of the rolling can be obtained, whereby the rolling schedule can be determined. The fine rolling carried out subsequent to the coarse rolling, however, is controlled not with the minimum rolling time as goal but with the fine crown, etc. as the parameters determining according to the rolling schedule. The rolling time is thus determined by the length of the material and the thickness reduction to be obtained by rolling. In other words, the rolling schedule for the fine rolling is set such as to provide a predetermined finish precision of the product.
The rolling schedule is thus set according to a certain rolling goal and the specifications of the rolling apparatus such as the rolling pressure may be satisfied, but the ratings of the motor for driving the roller are not always satisfied. This means that it is liable that the motor is continuously driven in an overloaded condition. In such a case, the motor coil insulation and lubrication would be deteriorated due to heat generation, which result in motor troubles. Further, in the case of the minimum time rolling, there is no guarantee that the minimum necessary power be supplied, so that the power efficiency is inferior.